Referring to FIG. 1, a gas turbine engine is generally indicated at 10 and comprises, in axial flow series, an air intake 11, a propulsive fan 12, an intermediate pressure compressor 13, a high pressure compressor 14, combustion equipment 15, a high pressure turbine 16, an intermediate pressure turbine 17, a low pressure turbine 18 and an exhaust nozzle 19.
The gas turbine engine 10 works in a conventional manner so that air entering the intake 11 is accelerated by the fan 12 which produce two air flows: a first air flow into the intermediate pressure compressor 13 and a second air flow which provides propulsive thrust. The intermediate pressure compressor compresses the air flow directed into it before delivering that air to the high pressure compressor 14 where further compression takes place.
The compressed air exhausted from the high pressure compressor 14 is directed into the combustion equipment 15 where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive, the high, intermediate and low pressure turbines 16, 17 and 18 before being exhausted through the nozzle 19 to provide additional propulsive thrust. The high, intermediate and low pressure turbine 16, 17 and 18 respectively drive the high and intermediate pressure compressors 14 and 13, and the fan 12 by suitable interconnecting shafts.
In view of the above, it will be appreciated that operation of a gas turbine engine is dependent upon a number of factors and it is desirable to operate that engine as efficiently as possible. A good measure of performance efficiency and health of a gas turbine engine is through specific fuel consumption (SFC) where SFC=fuel flow divided by net thrust from the engine. Unfortunately, with regard to gas turbine engines in service, that is to say with regard to an aircraft application on-wing, it is not possible to obtain a direct measurement of engine thrust. In such circumstances a traditional approach relates to monitoring changes in engine on-wing fuel burn through noted changes in measured fuel flow at measured engine power setting parameters. These power setting parameters are directly related to engine thrust and can be one of several parameters such as core engine pressure ratio, low pressure shaft speed or integrated core and bypass pressure ratio. In such circumstances, on-wing or in operation monitoring of the fuel flow to the gas turbine engine at a power setting parameter is used to give an indication of any deterioration in the efficiency of the engine. A specified power setting parameter is required as power transients will inherently involve changes in fuel flow rate.
Although monitoring of fuel flow at engine power settings gives a reasonable measure of the health of the engine and its efficiency it will be understood that the usefulness of this monitoring can be limited or diminished due to normal or expected engine deterioration. It will be appreciated that an engine deteriorates through wear in service and therefore the relationships between net thrust and power setting parameters can change slightly. These changes are small from the point of view of the integrity of engine power settings but significant relative to monitoring changes in fuel burn at the power setting to determine levels of efficiency. It will be appreciated that changes of 0.1% are generally considered significant with regard to monitoring fuel flow for efficiency. For example, with regard to core engine pressure ratio, engine degradation will cause the net thrust at the core engine pressure ratio to increase by in the order of 0.2 to 1.0%. In such circumstances the measured deterioration in engine fuel flow at the core engine pressure ratio will appear to be higher than the genuine deterioration in engine specific fuel consumption and so give a more pessimistic view of fuel consumption for the engine than is present in reality.